This invention relates to a system in which a host computer is connected through a connection-oriented local area network (LAN) to a plurality of terminals and particularly to establishing data links through the LAN between the host computer and terminals.
In a local area network, an OSI data link layer is divided into two sublayers, namely a media access control (MAC) sublayer and a logical line control (LLC) sublayer. The media access control sublayer is accessed either by CSMA/CD or by token passing. The logical line control sublayer serves in operation types 1 through 3. The operation type 1 is called an "unacknowledged connectionless service" and sets no data link. The operation type 2 is called a "connection-oriented service" and sets data links. The operation type 3 is called an "acknowledged connectionless service" and sets no data link. Any of the operation types 1 through 3 is applicable in common to the two different manners of accessing the media access control sublayer.
The CSMA/CD is recommended in ISO 8802/3 and is for using a bus-type local area network in common by a plurality of nodes, which are connected by a common medium and a medium attachment unit (MAU). Each frame of the media access control sublayer comprises eight fields, namely, (1) a preamble, (2) a frame start delimiter, (3) a destination address (DA), (4) a source adder (SA), (5) a data length, (6) information, (7) a PAD, and (8) a frame check sequence (FCS). Each of the destination and the source addresses is represented either by forty-eight bits or by sixteen bits. Each node of the media access control sublayer detects all frames transmitted through the common medium and receives only those of the frames which include the destination address coincident with an address assigned to the node under consideration.
Depending on the net topology applied, token passing is divided into two types, namely, (1) token ring and (2) token bus recommended in ISO 8802/4 and 8802/5. In accordance with the token passing, a token is circulated through the nodes. The token is control information which allows transmission of data. Like the frame used in CSMA/CD, each token includes the destination address and the source address. Each node monitors the token transmitted through a communication channel and receives the token which includes the destination address coincident with an address allotted to the node in question. When a node received the token and is going to transmit data, the node is allowed to use the communication channel.
A command and a response of the logical line control sublayer are the information transmitted by the media access control sublayer. Between a pair of link layer service access points (LSAP's) of transmitting and receiving stations, the logical line control sublayer transmits protocol data units (PDU's) according to logical line control (LLC). Each protocol data unit comprises (1) a destination address (DSAP address), (2) a source address (SSAP address), (3) a command, and (4) information. Each of these destination and source addresses is represented by eight bits. Unique discrimination is possible in the local area network by combinations of the address in the media access control sublayer (called MAC address) and the address in the logical line control sublayer (called LCC address). According to the operation type 2 of the logical line control sublayer, setup or establishment and clear or disconnection of a data link are carried out by an SABME command, PDU, a DISC command PDU, a UA response, and a DM response. Alternatively, the data link is set up and disconnected by using a unique sublayer of the media access control sublayer or an LAP-B defined as a unique logical line control sublayer by the CCITT.
In the manner described in the foregoing, a connection request and its acknowledgement of the logical line control sublayer are exchanged between two nodes which are assigned with addresses coincident with the destination address used in the media access control sublayer. More particular, one of the two nodes transmits the connection request to the other of the two nodes and the other of the two nodes sends the acknowledge response back to the above-mentioned one of the two nodes. A particular data link is set up for the first time after exchange of the connection request and its acknowledgement for an exclusive use. In general, a combination of the MAC address and an LLC address is referred to simply as a data link address. When the unique logical line control sublayer is used, the LLC address may not be used.
In a conventional data link setup system, a memory device of a host computer is preliminarily loaded with data link addresses of all terminals that are allowed to establish data links. When received from one of the terminals, a data link connection request is accepted if the data link address of the terminal under consideration is included as a source link address in the data link addresses stored in the memory device. If not included, the data link connection request is rejected. This is in order to allow setup of the data links only to the terminals having the data link addresses preliminarily selected and to prevent other undue terminals from accessing the host computer. It may, however, be mentioned here that new terminals can readily be added to a local area network and that an enormous number of new terminals are added when a plurality of local area networks are connected together. Consequently, the memory device must have a large memory capacity for storage of the data link addresses of such terminals. Furthermore, additional storage of the data link addresses of the new terminals may frequently occur. Depending on an on-line system in which the host computer is used, operation of the on-line system must be suspended on storing a new data link address in the memory device. In such an event, operation must be frequently suspended. This adversely affects performance of the on-line system.
In addition, network security must be taken into consideration. The network security is insufficiently insured if each terminal is given a right to access to the host computer. For example, attention will be directed to a program of carrying out on-line reference to and renewal of a data base connected to the host computer from a terminal. It is usual in this event to carry out user identification for a security check on a level of the program by preliminarily giving an individual user (operator) of the terminal a right of reference and/or renewals of the data base and by making the user send a user's name, an account name, a password, and the like. The network security, however, enables only a limited number of terminals to enjoy the service and results in accordingly deteriorated service. It is therefore desirable to restrict the network security to the least possible extent of access.
In a conventional on-line information processing system, the terminals deliver local connection requests to the local area network. An on-line program unit in the host computer is used as an information processing unit and has a program memory area for storing a control program for carrying out on-line information processing and in producing host connection requests indicative of the terminals. A data link control unit is connected to the local area network and the on-line program unit to control establishment of a plurality of data links on or through the local area network in compliance with the host connection requests from the on-line program unit to selected ones of the terminals and in compliance with the local connection requests from some of the terminals to the host computer.
In order to establish desired data links, the on-line program unit includes communication control information areas for storing fixed information for use in establishing the data links and for use in holding corresponding data link addresses indicative of the terminals, to and from which the data links should be established. The communication control information areas have therefore been equal in number to the terminals and have a large memory capacity in total.
It may be mentioned here furthermore that an on-line information processing arrangement is often used in an on-line information processing system in which the fixed information is stored in the communication control information areas on initializing the information processing system. In such an event, operation of the system has been suspended for making the system include a new terminal, to or from which an additional data link is to be established. This adversely affects performance of the system.